In a wireless communication system, a base station provides one or more coverage areas, such as cells or sectors, in which the base station may serve wireless communication devices (WCDs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality. Further, the base station may be in communication with network infrastructure including a gateway system that provides connectivity with a transport network such as the Internet for instance. With this arrangement, a WCD within coverage of the base station may engage in air interface communication with the base station and may thereby communicate via the base station and gateway system with various other entities.
In general, a base station may provide service in accordance with a particular air interface protocol or radio access technology, examples of which include Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), IEEE 802.11 (WiFi), and others now known or later developed.
In accordance with the air interface protocol, each base station may assign a certain extent of air interface channels for use in providing wireless service to WCDs. By way of example, each base station may provide wireless service to WCDs on one or more radio frequency carriers, where each carrier defines one or more ranges of frequency spectrum and has a respective downlink channel for carrying communications from the base station to WCDs and a respective uplink channel for carrying communications from the WCDs to the base station. Such carriers may be frequency division duplex (FDD), in which the downlink and uplink channels are defined as separate respective ranges of frequency, or time division duplex (TDD), in which the downlink and uplink channels are defined on a common range of frequency but distinguished through time division multiplexing. Further, the downlink and uplink channels may then define various sub-channels for carrying particular communications, such a control signaling and data (e.g., application layer data) between the base station and served WCDs. A given base station could be arranged to serve a WCD on a single such carrier at a time or, with “carrier aggregation service” or the like, on multiple carriers at a time.
Each carrier may also define various logical channels to facilitate communication between the base station and one or more served WCDs. For instance, on the downlink, a carrier may define a reference channel on which the base station broadcasts a reference signal useable by WCDs to detect and evaluate coverage, various other downlink control channels to carry control signaling (such as resource-scheduling directives) to WCDs, and one or more shared or traffic channels for carrying bearer data (e.g., user or application level data) to WCDs. And on the uplink, a carrier may define one or more uplink control channels to carry control signaling (such as resource scheduling requests) from WCDs, and one or more shared or traffic channels for carrying bearer data from WCDs. In practice, the shared or traffic channels may define particular physical resources for carrying data between the base station and WCDs.
When a WCD initially enters into coverage of such a system (e.g., powers on or moves into coverage of the system), the WCD may scan for a strongest coverage area in which to operate, and the WCD may then engage in signaling with the base station that provides that coverage area, to establish a radio-link-layer connection with the base station and to register the WCD to be served by the base station and generally by the network. Thereafter, the base station may serve the WCD with data communications on one or more carriers. For instance, when data such as user-plane (e.g., bearer) data and control-plane (e.g., control signaling) data arrives over the transport network for transmission to the WCD, the gateway system may transmit the data to the base station, and the base station may then transmit the data over the radio-link-layer connection to the WCD.